Feed forward amplifiers (“FFA”) are well known. Within the art, it is also known to improve the linear behavior of FFAs using a pilot tone. In simple terms, a pilot tone is generated by a pilot tone generator and injected into the signal path prior to the main amplifier. The injected pilot tone is then detected by a pilot tone receiver at the output of the circuit. The detected pilot tone is used to adjust the gain and phase of the output of the correctional amplifier, such that the pilot tone and error or distortion introduced by the main amplifier are substantially eliminated. The overall result is a substantially linear feed forward amplifier. More detailed explanations of pilot tone usage in FFAs can be found in a variety of sources, including U.S. Pat. No. 5,874,856 to Van Horn, the contents of which are incorporated herein by reference.
While the use of a pilot tone can improve the overall linearity of FFAs, current pilot tone generators and receivers tend to add complexity and overall cost to the FFA circuit. For example, current FFA circuits that utilize a pilot tone typically require multiple oscillators—i.e., a first oscillator for the pilot tone generator, and a second oscillator for the pilot tone receiver. Furthermore, these two oscillators must be programmed with complementary frequencies: namely, a first frequency for the first oscillator, and a second frequency for the second oscillator, whereby the second frequency differs from the first frequency by the intermediate frequency (“IF”) of the pilot tone receiver. Another problem with using two oscillators is that phase noise, (jitter in the time domain) can exist between the oscillators, thus requiring additional care, cost and/or complexity when designing both the generator and receiver to compensate for such phase noise.
Yet another problem with the prior art is that certain elements in the pilot tone receiver, such as the filter, can be difficult to appropriately size so that they properly complement the correctional amplifier and delay element used in the error signal path of the FFA circuit, thus increasing the overall complexity and cost of designing the FFA.